Fluid pressure operated motors

ABSTRACT

A fluid operated motor having a reciprocating piston with two opposed different effective areas of which the smaller effective area is open to continuous fluid pressure in a constant pressure chamber to pressure bias the piston in one axial direction, the larger effective area communicates with an alternating pressure chamber and in which reciprocation of the piston is achieved by alternately fluid pressurizing and exhausting the alternating pressure chamber by control of valve means which valve means further acts to open communication between the constant pressure and alternating pressure chambers when the piston is moving against its axial biasing so that fluid displaced from the constant pressure chamber enters the alternating pressure chamber. The motor also including a variable volume chamber which is open to exhaust and expands as the piston moves in its bias direction and contracts as the piston moves against its bias direction and the valve means still further acts to open communication between the variable volume chamber and alternating pressure chamber when the variable volume chamber is expanding and to close such communication when the variable volume chamber is contracting so that fluid from the contracting variable volume chamber is delivered to exhaust to alleviate intermittent exhaust back pressure in the motor. An embodiment of the invention includes the motor in a percussive device.

United States Patent 1191 Butterworth 1 1 Jan. 30, 1973 [54] FLUID PRESSURE OPERATED [57] ABSTRACT MOTORS A fluid operated motor having a reciprocating piston [75] Inventor: Philip Butterworth, Cumberland, with two opposed different effective areas of which England the smaller effective area is open to continuous fluid pressure in a constant pressure chamber to ressure [73] Asslgneg'. f t Hydrauhc Develop bias the piston in one axial direction, the larg r effecmems Lmmed London England tive area communicates with an alternating pressure [22] Filed: Aug. 27,1971 I chamber and in which reciprocation of the piston is achieved b alternate! fluid ressurizin and exhaust- [21] Appl' Noz'l'lsfiss ing the alt erna'ting pf essure chamber by control of valve means which valve means further acts to open 52 11.5. CI. ..91/231, 91/300, 91/321, communication between the constant pressure and 91 403 91 43 ternating pressure chambers when the piston is mov- 511 1m. 01 ..F0lb l/02,F01b 7/18, F01] 25/04 s against its axial biasing so that fluid displaced from 58] i ld of Search 91 23 300 321 403 43 the constant pressure chamber enters the alternating 230 pressure chamber. The motor also including a variable g volume chamber which is open to exhaust and ex- 5 References Cited pands as the piston moves in its bias direction and contracts as the piston moves against its bias direction UNITED STATES PATENTS and the valve means still further acts to open communication between the variable volume chamber and al- "91/23] ternating pressure chamber when the variable volume 3:552:26) 1 1971 Arndt ..91 300 chamber exPimdmg and m close Such commumca tion when the variable volume chamber is contracting FOREIGN PATENTS OR APPLICATIONS so that fluidv from the contracting variable volume chamber is delivered to exhaust to alleviate intermit- 65l,732 ll/l962 Canada ..9l/300 Primary Examiner- Paul E. Maslousky Attorney-Stevens, Davis, Miller &' Mosher tent exhaust back pressure in the motor. An embodiment'of the invention includes the motor in a percussive device.

16 Claims, 2 Drawing Figures PATENTEDJAN 30 I975 SHEET 1 BF 2 mmmrmmosrix O.

INVENTOR PHILIP BUTTERWORTH BY I I .MQWMVM ATTORNEYS PATENTEDJAN 30 ms 3.713.367

saw 2 [1F 2 FIG.2

Q TO ATMOSPHERE INVENTOR PHILIP BUTTERWORTH gymmww ATTORNEYS FLUID PRESSURE OPERATED MOTORS This invention relates to fluid pressure operated reciprocating motors.

The present invention is particularly concerned with fluid pressure operated motors which utilize a pressure biased piston axially slideable in a piston cylinder, the piston having different effective areas of which, the larger effective area defines with one part of the piston cylinder an alternating pressure chamber which is adapted to alternately communicate with fluid under pressure and with exhaust, and the smaller effective area defines with another part of the piston cylinder a constant pressure chamber which, when communicating with fluid under pressure, causes the piston to be biased in one direction. The constant pressure chamber is adapted to be in permanent communication with a source of fluid under pressure; consequently, by alternately pressurizing and exhausting the alternating pressure chamber, axial reciprocating movement of the piston in its cylinder is obtained. Fluid pressure operated motors of the kind having a pressure biased piston as aforementioned have previously been proposed, for example, as disclosed in German Patent specification Nos. 736,778 and 338,430, United Kingdom Patent specification No. 1,004,692 and Swedish Patent specification No. 141,366. However, each of the hitherto proposed motors has the inherent disadvantage that undesirable pulsation is created during operation, particularly if the piston has to cycle or reciprocate at high speed.

If a pressure biased piston is actuated for movement in its cylinder by a constant flow of fluid under pressure and the piston is required to move faster in one sense of axial direction than in the opposite sense, this is achieved by utilizing the different effective areas of the pressure biased piston for the two different directions. However, it also follows that the larger effective displaced volume (i.e. from the alternating pressure chamber) will have to exhaust at a high speed in comparison with the evacuation of the smaller effective displaced volume (i.e. from the constant pressure chamber). An example of this can be considered for a pressure biased piston which, in operation, has a 2 to 1 speed ratio in the axial direction. Therefore the rate at which fluid returns to exhaust on the fast stroke (i.e. with the alternating pressure chamber connected to exhaust) is ,twice the rate of the fluid delivery to the constant pressure chamber. Further, for the slow speed stroke (with the alternating pressure chamber connected to fluid under pressure), the rate of evacuation of fluid from the constant pressure chamber is half the speed of fluid delivery. It will be apparent that this provides a difference in fluid displaced-by the motor of4 to l for a motor having a pressure biased piston which has a 2 to 1 speed ratio. Consequently, the difference between the rate of flow of fluid into, and from, the motor creates the aforementioned undesirable pulsations. As the difference between the speed of reciprocation of the pressure biased piston in both senses of axial direction is made greater, so the difference between the rate of flow of fluid to exhaust will be increased thereby aggravating the undesirable pulse effect which is created at exhaust.

It is an object of the present invention to provide a cyclicly operating linear reciprocating fluid pressure operated motor, which includes a pressure biased piston axially slideable in a piston cylinder which motor, in operation with constant inflow rate of fluid under pressure alleviates substantial exhaust pulsation.

According to the present invention there is provided a fluid pressure operated reciprocating motor which includes a pressure biased piston axially slideable in a piston cylinder, the piston having different effective areas of which the larger effective area defines with one part of the piston cylinder an alternating pressure piston chamber and the smaller effective area defines with another part of the piston cylinder a constant pressure piston chamber which constant pressure piston chamber is in permanent communication with a fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the piston is biased for movement in one sense of axial direction; valve means adapted to connect alternately the alternating pressure piston chamber with fluid pressure inlet and exhaust to reciprocate the piston when the inlet is connected to a source of fluid under pressure, said valve means further being adapted to open communication between the constant pressure piston chamber and the alternating pressure piston chamber when the piston is moving in the direction against its bias so that fluid displaced from the constant pressure piston chamber is fed back into the alternating pressure piston chamber; a variable volume chamber which is in substantially continual communication with exhaust and is controlled by the piston so as to expand or contract as the case may be when the piston moves in the direction of or against its bias respectively; and wherein the valve means acts to close and open communication alternately between the alternating pressure chamber and the said variable volume chamber so that exhaust fluid from the alternating pressure piston chamber can enter the expanding said variable volume chamber when the piston is moving in the direction of its bias and that, when the piston is moving in the direction against its bias, fluid from the contracting said variable volume chamber is delivered directly to exhaust to alleviate intermittent exhaust back pressure in the motor.

The variable capacities of the three chambers are preferably so related that when the alternating pressure piston chamber is expanding, the volume of fluid displaced from theconstant pressure piston chamber in a given time is substantially equal to the volume of fluid entering the alternating pressure piston chamber less the volume of fluid displaced from the variable volume chamber in that time; and that when the alternating pressure piston chamber is contracting the volume of fluid entering the constant pressure piston chamber in a given time is substantially equal to the volume of fluid displaced from the alternating pressure piston chamber less the volume of fluid entering the variable volume chamber in that time.

.The motor of the present invention can include a pressure biased piston axially slideable in a piston cylinder, the piston having different effective end areas, of which the larger effective end area defines with one end of the cylinder an alternating pressure piston chamber and the smaller effective end area defines with the other end of the cylinder a constant pressure piston chamber. The constant pressure piston chamber is in permanent communication with a fluid inlet port. A secondary piston is associated with the pressure biased piston and is adapted to be actuated therewith. The secondary piston is axially slideable in a secondary cylinder and defines therewith a variable volume chamber which is in substantially continual communication with an exhaust port. The valve means can be provided by a single valve which is operatively controlled by movement of the piston and is adapted alternately to open and close communication between the alternating pressure piston chamber and the constant pressure piston chamber and concurrently to close and open respectively communication between the variable volume chamber and the alternating pressure piston chamber. The three chambers are so arranged that when the alternating pressure piston chamber is expanding or contracting, the constant pressure piston chamber and the variable volume chamber are concurrently both contracting or expanding respectively.

In one form of construction the motor of the present invention has a pressure biased piston and a pressure biased spool which mutually control reciprocation of each other, the spool, in effect, providing the valve means. The pressure biased piston operates in a piston cylinder under continuous pressure of fluid admitted to the constant pressure piston chamber and under alternating pressure and exhaust by way of a piston signalling passage to and from the alternating pressure piston chamber. The pressure biased spool likewise operates in a spool cylinder under continuous fluid pressure to a constant-pressure spool chamber which, when in communication with fluid pressure biases the spool in one sense of axial direction, one wall of which is a smaller effective area of the spool, and under alternating fluid pressure and exhaust by way of spool signalling passage means to and from an alternatingpressure spool chamber, one wall of which is a larger effective area of the spool. The piston is conveniently arranged to define with its cylinder three axially spaced chambers, the volume of each of which is variable by axial movement of the piston in the piston cylinder; one end chamber being the alternating pressure piston chamber, the intermediate chamber being the constant-pressure piston chamber and the other end chamber being the variable volume chamber which is substantially continually open to exhaust. Movement of the spool in the direction of its bias serves to open communication between a fluid inlet and the piston signalling passage and also to close communication between the exhaust and the piston signalling passage, and vice versa; Movement of the piston in the direction of its bias serves to open communication, by way of the spool signalling passage means, between the alternating pressure spool chamber and the variable volume chamber and hence exhaust, while movement of the piston in the direction against its bias serves to open communication, by way of the spool signalling passage means, between the alternating pressure spool chamber and the. constant pressure piston chamber and hence the fluid outlet. The three chambers in the piston cylinder are so arranged that when the alternating pressure piston chamber is expanding or contracting the constant pressure piston chamber and the variable volume chamber are both respectively contracting or expanding.

Preferably the piston reciprocates in a piston cylinder having a stepped bore and carries two piston lands, the first, which is located in the larger bore of the piston cylinder, has differential effective end areas and constitutes the pressure-biased piston proper, while the second, which is located in the smaller bore of the piston cylinder, may be considered as constituting a pumping piston. There are threechambers in the piston cylinder, viz

i. An alternating pressure piston chamber defined by the end of the larger piston cylinder bore and the outwardly facing end of the first piston land;

ii. A variable volume chamber defined by the end of the smaller piston cylinder bore and the outwardly facing end of the second piston land;

iii. A constant pressure piston chamber constituted by the annulus defined by the inwardly facing end of the first piston land and the part of the smaller bore of the piston cylinder sealed by the second piston land. The spool is conveniently pressure biased and can carry two spool lands, to form three chambers in the spool cylinder viz:

i. an alternating-pressure spool chamber formed in one part of the spool cylinder and communicating with a larger effective area of the spool;

ii. a constant-pressure spool chamber formed in a further part of the spool cylinder and communicating with a smaller effective area of the spool; and

iii. a movable transfer chamber formed in a still further part of the spool cylinder.

The motor has a fluid inletport; permanently open pressure passages connecting the fluid inlet port with the movable constant-pressure piston chamber and with the constant-pressure spool chamber; and a fluid exhaust port. Permanently open exhaust passages connect the fluid exhaust port with the movable transfer chamber and with the variable volume chamber. The motor further includes a piston signalling passage, including a port adapted to be opened or closed alternately to inlet and exhaust bymovement across it of a first spool land, thus alternately connecting with the alternating pressure piston chamber with the fluid inlet port, or with the movable transfer chamber and hence with the fluid exhaust port; and spool signalling passage means including ports in the piston cylinder adapted to be opened and closed alternately to inlet and exhaust by movement across it of the second piston land and alternately connecting the alternating pressure spool chamber with the constant-pressure piston chamber and hence with the fluid inlet port, or with the said variable volume chamber and hence with the fluid exhaust port. The arrangement of the motor is such that fluid displaced from the constant-pressure piston chamber (when the first piston land is moved by pressure in the alternating-pressure piston chamber) can feed back into thealternating-pressure piston chamber by way of the piston signalling passage, while fluid displaced from the contracting said variable volume chamber passes to exhaust and fluid displaced from the alternatingpressure piston chamber (when the first piston land is moved by pressure in the constant pressure piston chamber) can feed into the expanding variable-volume chamber and partly to exhaust by way of the piston signalling passage and the movable transfer chamber.

To alleviate the possibility of the motor coming to rest or being assembled in a stalled condition, the ports in the piston cylinder and in the spool cylinder and lands of the piston and spool which respectively control the ports are preferable arranged in accordance with the disclosure in British Patent specification No. l l57440.

Conveniently, to provide the pressure-biased spool, the spool cylinder is formed with a stepped bore and a spool rod having two spool lands is utilized the first land of the spool being located in the larger bore of the spool cylinder and having its outwardly facing end constituting the larger of the differential effective areas of the spool while the second land is located in the smaller bore of the spool cylinder and has its outwardly facing end constituting the smaller of the differential effective areas of the spool. In this case the alternating pressure spool chamber is defined by the end of the larger spool cylinder bore and the outwardly facing larger effective area of the first spool land, the constant pressure spool chamber is defined by the end of the smaller spool cylinder bore and the outwardly facing smaller effective area of the second spool land, and the movable transfer chamber is formed by the annulus defined by the inwardly facing end of the first spool land and the part of the smaller bore of the spool cylinder sealed by the second spool land.

In the motor of the present invention, the mechanical output can be taken from either the piston or the spool. However, in practice, the mechanical output is usually derived from the pistonas, firstly, when the motor is in operation, the piston exhibits constant reciprocation compared to the spool which exhibits intermittent reciprocation, i.e., the time taken for the piston to reverse can be considered negligible in comparison with the time taken for the spool to reverse; secondly the piston is usually constructed of a more robust nature than the spool and is often larger in diameter than the spool so that the areas of the piston on which the fluid pressure acts provide a large power output. The mechanical output from the piston (and the spool if required) is conveniently taken through a piston rod which extends axially from an end of the piston, through the piston cylinder and slideable in sealed manner through the wall of the motor which defines the piston cylinder. The piston is conveniently provided with two piston rods each extending axially through the piston cylinder one from each end of the piston, both piston rods extending in sealed and axially slideable manner through the wall of the motor which defines the piston cylinder.

In certain instances it may be desirable to have rods extending from'both the spool and piston in which, during operation, the rods from the piston can provide a power drive and the rods from the spool can be used as a signal, for example, to initiate operation of auxiliary apparatus.

The term fluid as used throughout this specification is intended to include both gases and liquids. It is to be appreciated, however, that the motor is particularly directed towards the alleviation of intermittent exhaust back pressure (exhaust pulsation) when the motor is operated by pressure liquid and although such exhaust pulsation is unlikely to be detrimental or even encountered by use of the motor with gas under pressure, such use can be made of the motor and any minor modifications, for example, modified sealing arrangements which prove necessary will be apparent to persons skilled in the relevant art.

Although the variable volume chamber is in substantially continual communication with exhaust and is controlled by the piston so as to expand or contract, such communication of the variable volume chamber with exhaust may be cut off by the piston at or towards the end of its stroke in a sense to contract the variable volume chamber. In this way the fluid may be trapped and compressed in the variable volume chamber by the piston at the end of stroke of the piston in a sense to contract that chamber and this fluid can be used with advantage to provide hydraulic arresting of the piston and thereby prevent the latter from continually abutting against the respective end of the piston cylinder. Similarly when the valve means comprises a spool valve, the spool may be so arranged that at or towards each end of its stroke it closes an outlet from a chamber (which outlet preferably communicates with exhaust) and in so doing traps and compresses a volume of fluid in such chamber to cushion or provide hydraulic arresting for the spool at its end of stroke. Conveniently the hydraulic arresting of the spool in each sense of direction is achieved by fluid trapped in a chamber formed between the end of a spool rod which extends at an end of the spool and the closed end of a complementary bore in which the spool rod is slideable.

One embodiment of the present invention will now be described, by way of example only, with reference to the accompanying illustrative drawing in which FIG. 1 is an axial cross section through a fluid pressure operated motor constructed according to the present invention and which incorporates a pressure biased piston and a pressure biased spool; and

FIG. 2 is a similar view to that shown in FIG. 1 and illustrates the control and relative positioning afforded by the piston and spool during a cycle of operation of the motor.

The motor includes a body 15 which houses a pressure biased piston shown generally at 17 and a pressure biased spool shown generally at 16, which are co-axially disposed with respect to each other and both of which are axially slideable for a reciprocatory stroke in the body 15. The piston 17 is axially slideable within a piston cylinder 17a having a stepped bore. The piston 17 is provided with two axially spaced lands 17b and of which the land 17b is slideable within the larger diameter bore of the cylinder 17a and the land 170 is slideable within the smaller diameter bore of the cylinder 17a. Extending axially, one from each end of the piston 17, are a pair of piston rods 20 and 21 which are respectively slideable in bores 20a and 21a in the body 15. The piston rod 20 passes through its adjacent side wall of the body 15 (which side wall partly defines the cylinder 17a) and is capable of axial sliding movement therethrough in sealed manner. The diameter of the piston 17 for the part of its length between two lands 17b and 17c is less than the diameter of the piston land 17c and provided that the diameter of the land 17c is greater than the diameter of the rods 20 and 21 and is less than the diameterof the land 17b, the part of the piston 17 for its length between the two lands 17b and 170 can be of any diameter less than 170. Consequently the effective area of the side of the land 17b adjacent to the piston rod 20 is larger than the effective area of the side of the land 17b adjacent to the land 17c. Consequently the first land 17b has differential effective areas and constitutes the pressure biased piston proper, while the second land 17c can be considered as constituting a pumping piston. The piston 17 defines with its cylinder 17a three chambers; an alternating pressure piston chamber 12 which is defined by the end part of the larger diameter bore of the cylinder 17a and the adjacently facing larger effective area of the first piston land 17b; a variable volume chamber 14 which is defined by the end part of the smaller diameter bore of the cylinder 17a and the adjacently facing end of the second piston land 17c; and a constant pressure piston chamber 13 which is formed in the piston cylinder 17a between the lands 17b and 170.

The pressure-baised spool 16 is axially slideable within a spool cylinder 16a having a stepped bore. The spool 16 comprises a spool rod 16b, l6e which carries three axially spaced lands 16c, 16d and 16f. The lands 16c and 16f are located in the larger bore of the spool cylinder 16a and the land 16f has its side face remote from the land 16c constituting the larger of the differential effective areas of the spool, while the land 16d is located in the smaller bore of the spool cylinder 16a and the land 16d has its side face remote from the land 16c constituting the smaller of the differential effective end-areas of the spool. The spool 16 effectively defines with the spool cylinder three chambers; an alternating pressure spool chamber 18 part of which is defined by the end of the larger diameter bore of the spool cylinder 16a and the outwardly facing larger effective area end of the spool land 16f; a constant pressure spool chamber 11 defined by the end of smaller diameter bore of the spool cylinder 16a and the outwardly facing smaller effective area end of the spool land 16d; and a transfer chamber which is formed in the spool cylinder 16a between the lands 16c and 16d. It will be seen that the alternating spool chamber 18 is effectively formed in two parts, one on each side of, the spool land 16f, which two parts (shown at 9 and 18) are in permanent communication by way of passages 8 formed within the spool land 16f. Axially extending from the outwardly facing ends of the land 16d and 16f are spool rods 22 and 22a which are slideable in complementary blind bores 22' and 22a respectively. The ends of the spool rods 22 and 22a define with their respective blind bores arresting chambers a and 15b respectively. The arresting chambers 15a and 15b are capable of communicating with an exhaust port 24 through a main exhaust passage 2 by way of arresting ports 15a and 15b respectively which are located in the bore 22' and 22a and are controlled by the respectively associated spool rods 22 and 22a.

The constant pressure spool chamber 11 is in permanent communication through a, main pressure passage 1 with a fluid pressure inlet port 23. The constant pressure piston chamber 13 is similarly in permanent communication with the fluid inlet port 23 by way of a passage 4 and the passage 1.

Communicating between the spool cylinder 16a and the alternating pressure piston chamber 12 is a piston signalling passage 5 having a port 5a in the spool cylinder 16a which is adapted to be controlled by the spool land 16d. The passage 5 opens into the alternating pressure piston chamber 12 by way of both a port 5b and a restricted port 50 axially spaced therefrom; the port 5b is located at a position so that it can be closed by piston land 17b whereas the restricted port 5c is located at a position in which it is in permanent communication with the alternating pressure piston chamber 12. The transfer spool chamber 10 is in permanent communication with the exhaust port 24 through a branch of the main exhaust passage 2.

The variable volume chamber 14 is in substantially continual communication with the exhaust port 24 by way of a port 3. The phrase substantially continual communication is used since the port 3 can be closed for a brief period by the piston land 17c towards the extreme end of stroke of the piston 17 in the direction of arrow Y.

The bore 21a in which piston rod 21 is slideable has a vent port 21a which opens to atmosphere. The vent port 21a is intended to relieve any back pressure which may otherwise develop between the end of piston rod 21 and its bore due to fluid leakage past the piston rod from the variable valve chamber 14.

Communicating between the spool cylinder 16a and piston cylinder 17a are a pair of spool signalling passages 6 and 7 having axially spaced ports 6a and respectively in spool cylinder 16a and axially spaced ports 6b and 7b respectively in the piston cylinder 17a. The port 6a in the spool cylinder 16a is controlled by the spool land 16c and the port 7a in the spool cylinder is controlled by the land 16f. The ports 6b and 7b in the piston cylinder 17a are controlled by the piston land 170. The axial length of the second piston land is such that it never fully closes ports 6b and 7b simultaneously. Similarly the axial distance between the spool lands 16c and 16f is such that the ports 6a and 7a are never closed simultaneously by the spool so that one or the other of the ports 6a and 7a is always open to the alternating pressure spool chamber 9. Further, the spool lands 16c, 16d and 16f are so disposed relative to the ports 5a, 6a and 7a that when port 5a is closed by the spool land 16d both ports 60, 7a are open to communicate with the alternating pressure spool chamber 9.

We will now consider a cycle of operation for the motor constructed in accordance with the above description in which the motivating power is hydraulic fluid under pressure.

The free end of piston rod 20 is suitably connected to apparatus which is to be reciprocated or impacted and the port 23 connected to a source of hydraulic fluid under pressure. Fluid passes through inlet passage 1 into the constant pressure spool chamber 11 and also passes through passage ,4 into the constant pressure piston chamber 13 in both of which chambers fluid pressure is always present during a working cycle. This results in the piston 17 being biased for axial movement in the direction of Arrow X and the spool 16 being biased for axial movement in the direction of Arrow Y.

Port 24 is connected to exhaust which, in the present example where hydraulic fluid is used, is conveniently a return line to the fluid pump or reservoir. It is in this return line where intermittent exhaust back pressure or pulsation has hitherto been encountered.

In FIG. 1 the spool 16 is shown in the extreme right hand position at one end of its stroke in cylinder 16a. In this position the port 50 of the piston signalling passage 5 is open to the constant pressure spool chamber 11 and consequently fluid under pressure is present in the alternating pressure piston chamber 12. Since the effective area of the first piston land 17b in the alternating pressure piston chamber 12 is greater than the effective area of the piston land 17b in the constant pressure piston chamber 13, the piston 17 moves in the axial direction against its bias (i.e. in the direction Y). For the position of the piston and spool shown in H0. 1 the port 6a of spool signalling passage 6 is closed by the spool land 16c and the piston land 17c is moving in direction Y to close ports 6b. When port 6b is even tually closed by piston land 170 it is without effect since port 60 is closed by spool land 160. However, the alternating pressure spool chamber 18 and 9 is open to exhaust by way of port 7a, passage'7, port 7b, the variable volume chamber 14, port 3 and main exhaust passage 2; consequently the spool 16 is retained at the end ofits stroke in the direction of its bias (arrow X) by fluid pressure in the constant pressure spool chamber 11 acting on the smaller effective area of the spool.

As the piston 17 continues its movement in the direction of arrow Y, the second piston land 17c cracks open port 6b to fluid pressure by way of the constant pressure piston chamber 13 immediately prior to closing port 7b to exhaust whilst the port 6a remains closed by the spool land 160.

Further progress of piston 17 in the direction of arrow Y causes the piston land 170 to fully close port 7b to exhaust and reopen it to fluid under pressure by way of the constant pressure piston chamber 13, passage 4, the constant pressure spool chamber 11 and main pressure passage 1. Consequently fluid under pressure enters the alternating pressure spool chamber 18 and 9 and through the internal passages 8 in the spool land 16fto the end of the spool chamber 16a. As the effective area of the spool in the alternating spool chamber 18 and 9 is greater than the effective area of the spool in the constant pressure spool chamber 11, the spool 16 is lifted from its seat and moves in the direction Y against its bias to enlarge the chamber 18 formed with the end of the spool cylinder 16a. As previously mentioned, by providing the internal passages 8 which communicate between the chambers on both sides of the spool land 16f, the chamber 18 can in effect be considered as part of the alternating pressure spool chamber 9. If required, the end of the spool land 16f adjacent in the end of the spool cylinder 16a can be castellated to facilitate the flow of fluid over the end of the spool and the initial lifting of the spool from its seat. During initial movement of the spool in the direction Y the spool land 16c opens port 6a prior to port 7a being closed by spool land 16f thereby permitting further fluid under pressure to pass from the constant pressure piston chamber 13 into the alternating pressure spool chamber 9 (and 18) by way of port 6a to complete the axial movement of the spool to the end of its stroke in the direction Y.

As the spool moves in direction Y fluid in the arresting chamber b is displaced through port 15b to exhaust 24 until eventually the port 151) is closed by the spool rod 22a immediately before the spool completes its movement. When the port 15b is closed fluid trapped in arrestingchamber 15b is compressed by the spool and acts to cushion or hydraulically arrest the spool at its end of stroke. Simultaneously with the port 15b being closed by spool rod 22a the port is being opened by spool rod 22 to connect the arresting chamber 15a with exhaust 24. At the end of stroke of the spool 16 in direction Y the port 7a is closed by land 16]". During movement of the spool in the direction Y the spool land 16d closes port 5a of the piston signalling passage 5 to'the constant pressure spool chamber 11 (and consequently to fluid under pressure) and reopens the port 5 a to the transfer spool chamber 10 (and consequently to exhaust through the main exhaust passage 2) which opens the alternatingpressure piston chamber 12 to exhaust 24'. As fluid under pressure is always present in the constant pressure piston chamber 13 and the alternating pressure piston chamber 12 is open to exhaust, the piston 17 reverses and commences to move in the direction X of its bias (see FIG. 2).

As the piston 17 moves in the direction X the port 7b is closed. by the piston land to pressure (by way of the constant pressure piston chamber 13) and further movement of the piston causes the land 170 to close the port 6b whilst the port 7b is simultaneously being cracked open to exhaust (by way of the variable volume chamber 14). However, the system remains unaltered as the port 7a of the spool signalling passage 7 is closed by the land 16f. As will be apparent from the drawings, when both spool signalling passages 6 and 7 are closed the alternating pressure spool chamber 9 (and 18) is effectively sealed thereby providing a hydraulic lock which ensures that the spool 16 remains seated at the end of its stroke in the direction Y against its bias.

Still further movement of the piston 17 in the direction X causes piston land 17c to open port 6b to the variable volume chamber 14 and consequently to exhaust by way of port 3 and the main exhaust passage 2. Consequently the alternating pressure spool chamber 9 (and 18) is opened to exhaust and fluid under pressure in the constant pressure spool chamber 11 causes the spool 16 to lift from its seat and to move in the direction X. ln so doing the spool land 16d closes port 5a to exhaust (by way of the transfer spool chamber 10) and reopens the port 5a to fluid under pressure (by way of the constant pressure spool chamber 11). This movement opens the alternating pressure piston chamber 12 to fluid under pressure and the piston 17 reverses to move in the direction of arrow Y against its bias. lnitial movement of the spool 16 in the direction Y causes the spool land 16f to open port 7a to ensure that the alternating pressure spool chamber 9 (and 18) remains open to exhaust after the spool land 16 c has closed the port 6a when the spool reaches the end of its stroke in the direction of its bias (arrow X). The spool 16 moves in the direction of arrow X until it reaches the end of its stroke in its biased direction and with the piston 17 moving in the direction Y against its bias the conditions are reached as illustrated in FIG. 1 and the motor commences a further cycle.

It will be noted that as the spool moves in direction X fluid in the arresting chamber 15a is displaced through port 15a to exhaust it until eventually the port 15a is closed by the spool rod 22 immediately before the spool completes its movement when the port a is closed, fluid trapped in arresting chamber 15a is compressed by the spool and acts to cushion or hydraulically arrest'the spool at its end of stroke. Simultaneously with theport 15a being closed by spool rod 22, the port 15b is being opened by spool rod 220.

We will now consider in detail the features of the motor which are effective to alleviate intermittent exhaust back pressure or pulsation in the main exhaust passage 2 and the return line (not shown).

If the piston rods and 21 are of equal diameter, it follows (omitting stroke of the piston 17 since this is a common factor for the three chambers 12, 13 and 14) that the effective variation in volume for a given stroke in the alternating pressure piston chamber 12 is. equal to the sum of the effective variation in volumes for that stroke in the constant pressure piston chamber 13 and the variable volume chamber 14. When the piston is moving in the direction of its bias (arrow X) both the constant pressure piston chamber 13 and the variable volume chamber 14 are simultaneously expanding whilst the alternating pressure piston chamber 12 is contracting. With the piston moving in the direction X the spool (which can effectively be considered as valve means) causes the alternating pressure piston chamber 12 to be inter-connected by way of piston signalling passage 5, transfer spool chamber 10, passage 2 and port 3 with the variable volume chamber 14, whilst the constant pressure piston chamber 13 is connected to fluid under pressure. Consequently a volume of fluid equal to that required by the expanding variable volume chamber 14 flows from the alternating pressure piston chamber 12 to the chamber 14, Since the effective displaced volume from the alternating pressure piston chamber 12 exceeds that of the variable volume chamber 14 by an amount equal to the effectivevolume of fluid required by the constant pressure piston chamber 13, the excess fluid passes through the main exhaust passage 2 and down the return line to the fluid pump. Consequently it is seen that fluid is discharged from the motor through the exhaust port 24 at a rate equal to that by which the constant pressure piston chamber 13 is being charged from the inlet port 23.

When the piston 17 is moving in the direction Y against its bias (as is best seen in FIG. 1) the alternating pressure piston chamber 12 is expanding while the constant pressure piston chamber 13 and the variable volume chamber 14 are simultaneously contracting. In this case the spool 16 causes the alternating pressure piston chamber 12 to be charged by the full volume from the fluid pump through the main pressure passage 1, the constant pressure spool chamber 11 and the piston signalling passage 5. The volume of fluid which is displaced by the contracting constant pressure piston chamber 13 is recirculated to the alternating pressure piston chamber 12 by way of passages 4 and l, the constant pressure spool chamber 11 and piston signalling passage 5. As the effective charging volume for the alternating pressure piston chamber 12 less the effective (recirculated) displaced volume of the constant pressure piston chamber 13 is equal to the effective volume discharged from the contracting variable volume chamber 14, the rate at which fluid is being displaced from the variable volume chamber 14 through the main exhaust passage 2 is equal to the flow rate of the pump. It will be apparent that for each direction of axial movement of the piston 17, the flow rate of return of fluid flow through the exhaust port 24 will be substantially the same as the flow rate of fluid flow through the inlet port 23 regardless of the diameter of the variable volume chamber 14 (provided that the piston land 17b is pressure biased in the sense above described). To alter the relative speed between the forward and return strokes of the piston 17 (assuming that the rate of supply of fluid under pressure is constant) it is necessary only to alter the smaller effective area of the first piston land 17b which communicates with the constant pressure piston chamber 13. Such alteration can be effected by altering the diameter of the variable volume chamber 14 (Le. by varying the two relative diameters of the stepped bore of the piston cylinder 17a) whilst retaining constant return of fluid flow rate.

The manner in which the motor of the present invention alleviates intermittent exhaust back pressure in the flow line return will be appreciated from the following mathematical consideration which specifically relates to the fluid pressure operated motor as above described and illustrated.

In the drawing, let A1 be the cross-sectional (taken radially) area for each of the piston rods 20 and 21;

A2 be the cross-sectional (taken radially) area of the piston land 17b; and

A3 be the cross-sectional (taken radially) area of the piston land 17c.

Omitting stroke of the piston 17 as this is a common factor,

Volume of hydraulic fluid required to move the piston 17 leftwardly'in the drawing (in the direction of arrow Y see FIG. 1)

01(A2-Al) Volume of fluid displaced from constant pressure piston chamber 13 to alternating pressure piston chamber 12 (see FIG. 1)

a (AZ-A3) I Therefore volume of fluid required from pump a (AZ-A1) (AZ-A3) a (A3-Al) It will be apparent that the volume of fluid displaced from the contracting variable volume chamber 14 through port 3 and main exhaust passage 2 back to the P p 04 (A3-Al) Therefore volume of fluid displaced from the motor to exhaust equals the volume of fluid from the pump to the motor.

Further the volume of fluid required from the pump to move a piston 17 rightwardly in the drawing (ie in the direction of arrow X) a (AZ-A3) Volume of fluid displaced from the contracting alternating pressure piston chamber 12 a (AZ-A1) Volume of fluid absorbed by expanding variable volume chamber 14 a (A3-Al) Therefore volume of fluid which passes through the exhaust port 24 and flows back to the pump a (AZ-Al (A3-A 1) a(A2-A3) position in direction Y (see FIG. 2), for at least the majority of the stroke of the piston the port 3 is open to communication with the chamber 14 (hence the use of the phrase substantially continual communication of i the chamber 14 with exhaust) but if required the port 3 can be located to be in permanent communication with the variable volume chamber 14. However, by locating the port 3 in the position illustrated it will be apparent that if the piston land 17c closes this port as the piston approaches its end of stroke in the direction Y (for example due to the inertia of a heavily weighted piston), fluid trapped in the end of bore 21a by the piston rod 21 is compressed by the inertia of the piston and acts to cushion or hydraulically arrest the piston at its end of stroke.

Further, although the piston signalling passage 5 normally communicates with the alternating pressure chamber 12 by way of port 5b and restricted port 50, it is possible that the port 5b can be closed by piston land 17b as the piston approaches its end of stroke in the direction X. (For example if the piston has a high inertia or is heavily loaded in direction X). In the event of port 5b being closed, fluid is displaced from the chamber 12 solely by way of restricted port 50 (which is in permanent communication with chamber 12) which imparts a throttling action to displacement of the fluid and causes the fluid in chamber 12 to apply a degree of cushioning or hydraulic arresting to the piston at its end of stroke.

Several modifications can be made to the motor as above described and illustrated, for example, the spool valve can be replaced by another suitable form of control valve. If a spool is utilized it need not be of the stepped-bore type but can be pressure biased by the different effective areas of the spool being formed by axially extending through rods" in a similar manner to that above described and illustrated for the piston. The variable volume chamber l4-and associated piston pump which is effectively'formed by the land 17c of the piston can be housed in a separate body provided that contraction and expansion of the variable volume chamber is consistent with the expansion and contraction respectively of the alternating pressure piston chamber, and in fact a piston and cylinder type variable volume chamber can be replaced by a suitable bellows or other type of expandible chamber.

Generally all the ports in the spool and piston cylinders (as well as in the spool rod and piston rod bores) will be radially disposed and several in number (or comprise annular recesses) to avoid hydraulic side loadings on the spool and piston.

It will be apparent from the description with reference to the drawings that there is no relative positioning of the spool and piston in their respective chambers which results in the motor being in a stalled condition. This partly results from the provision of the pair of spool signalling passages 6 and 7 and the axial spacing of their ports 6b, 7b relative to the axial length of the piston land 17c and of their ports 6a, 7a relative to the spool lands 16c, 16d and l6fand the port 5a. The axial length of the land 17:: and of the spool lands is such that the alternating spool chamber 9 is always at least partially open to pressure or to exhaust when the port 5a of the piston signalling passage 5 is closed by the spool land 16d and consequently the spool will always move to one end of its stroke thereby ensuring that the alternating piston chamber 12 will always open to fluid under pressure or to exhaust, i.e., there is no condition for the motor in which the alternating pressure piston and spool chambers (12 and 9 respectively) are simultaneously closed either to exhaust or fluid pressures The motor as above described with reference to the drawings is particularly suitable for use as a percussive device in which the end of piston rod 20 is intended to act as a reciprocating hammer and impact (either directly or indirectly) a required tool or other component which can be separate from, or carried by, the device. For example, in FIG. 1 the body 15 is shown extended to provide (or has coupled thereto) a socket part in which the piston rod 20 reciprocates. The socket part 100 is intended to accommodate a shank 101 of a tool shown generally at 101a (for example a chisel, spade, moil, percussive drill bit or the like) the end of which shank is impacted by the end of the piston rod 20. The shank 101 is slideably mounted in the socket part 100 to be capable of axial movement therein and is removably retained by a pin 102 which is carried by the socket part and engages with a recess 103 in the shank to permit restricted axial movement of the tool relative to the socket part.

I claim:

1. A fluid pressure operated reciprocating motor which includes;

a pressure biased piston axially slidable in a piston cylinder, the piston having different effective areas of which the larger effective area defines with one part of the piston cylinder an alternating pressure piston chamber and the smaller effective area defines with another part of the piston cylinder a constant pressure piston chamber which constant pressure piston chamber is in permanent communication with a fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the piston is biased for movement in one sense, of axial direction;

valve means adapted to connect alternately the alternating pressure piston chamber with fluid pressure inlet and exhaust to reciprocate the piston when the inlet is connected to a source of fluid under pressure, said valve means further being adapted to open communication between the constant pressure piston chamber and the alternating pressure piston chamber when the piston is moving in the direction against its bias so that fluid displaced from the constant pressure piston chamber is fed back into the alternating pressure piston chamber; a variable volume chainber which is in substantially continual communication with exhaust and is controlled by the piston so as to expand or contract as the case may be when the piston moves in the direction of or against its bias respectively; and wherein the said valve means acts to close and open communication alternately between the alternating pressure chamber and the said variable volume chamber so that exhaust fluid from the alternating pressure piston chamber can enter the expanding said variable volume chamber when the piston is moving in the direction of its bias and that, when the piston is moving in the direction against its bias, fluid from the contracting said variable volume chamber is delivered directly to exhaust to alleviate intermittent exhaust back pressure in the motor.

2. A motor as claimed in claim 1 wherein the variable capacities of the constant pressure piston chamber, the alternating p ressure piston chamber and the said variable volume chamber are so related that when the alternating pressure piston chamber is expanding, the volume of fluid displaced from the constant pressure piston chamber in a given time is substantially equal to the volume of fluid entering the alternating pressure piston chamber less the volume of fluid displaced from the said variable volume chamber in that time; and that when the alternating pressure piston chamber is contracting the volume of fluid entering the constant pressure piston chamber in a given time is substantially equal to the volume of fluid displaced from the alternating pressure piston chamber less the volume of fluid entering the said variable volume chamber in that time.

3. A motor as claimed in claim 1 wherein the said variable volume chamber is defined by a secondary piston and cylinder device which device is operatively connected to the main piston and cylinder device which defines the constant and alternating pressure piston chambers to provide required sequential expansion and contraction of the said variable volume chamber in accordance with expansion and contraction of the constant pressure piston chamber during axial movement of the main piston in its cylinder.

4. A motor as claimed in claim 1 wherein the piston defines with its cylinder the said variable volume chamber.

5. A motor as claimed in claim 4 wherein the piston cylinder has a stepped bore and the piston carries two piston lands, the first, which is located in the larger bore of the piston cylinder, has differential effective end areas and effectively constitutes a part of the pressure biased piston proper, and the second, which is located in the smaller bore of the piston cylinder, effectively constitutes part ofa pumping piston; and wherein the alternating pressure piston chamber is formed betweenthe end of the larger piston cylinder bore and the outwardly facing end of the first piston land, the said variable volume chamber is formed between the end of the smaller piston cylinderbore and the outwardly facing end of the second piston land, and the continuous pressure piston chamber is constituted by the annulus defined by the wall of the stepped bore between the inwardly facing end of the first piston land and the opposed face of the second piston land.

6. A motor as claimed in claim 1 wherein the said valve means is provided by a single valve which is operatively controlled by movement of the piston and is adapted to alternately open and close communication between the alternating pressure piston chamber and the constant pressure piston chamber and concurrently to respectively close and open communication between the said variable volume chamber and the alternating pressure piston chamber.

7. A motor as claimed in claim 1 wherein the said valve means comprises a spool which reciprocates in a spool cylinder and in which the spool and piston mutually control reciprocation of each other in their respective cylinders.

8. A motor as claimed in claim 7 wherein a land of the spool controls the opening and closing of a port in the spool cylinder which is in permanent communication by way of a piston signalling passage with the alternating pressure piston chamber and movement of the spool is operatively associated with movement of the piston so that when the spool is located at one end of its stroke the piston signalling passage communicates by way of the spool cylinder with fluid pressure inlet whereby the piston is caused to move in the direction against its bias and when the spool is located at the other end of its stroke the piston signalling passage communicates by way of the spool cylinder both with exhaust and with the said variable volume chamber whereby the piston is caused to move in the direction of its bias.

9. A motor as claimed in claim 7 wherein the said valve means comprises a pressure biased spool axially slidable in the spool cylinder, the spool having different effective areas of which the larger effective area defines with one part of the spool cylinder an alternating pressure spool chamber and the smaller effective area defines with another part of the spool cylinder a constant pressure spool chamber which constant pressure spool chamber is in permanent communicationwith fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the spool is biased for movement in one sense of direction; and wherein the alternating pressure. spool chamber is adapted to communicate by way of spool signalling passage means with port means controlled by movement of the piston which port means is, during reciprocation of the piston, controlled to communicate alternately with fluid inlet and exhaust so that the alternating pressure spool chamber is alternately pressurized and exhausted to reciprocate the spool.

10. A motor as claimed in claim 9 wherein the port means of the spool signalling passage means is controlled by a land of the piston so that at one end of the stroke of the piston said port means communicates with fluid inlet and at the other end of the stroke said port means communicates with exhaust by way of the said variable volume chamber.

1 l. A motor as claimed in claim 9 wherein at one end of stroke of the piston, the port means of the spool signalling passage means communicate with fluid inlet by way of the constant pressure piston chamber.

12. A motor as claimed in claim 1 wherein the said variable volume chamber is in substantially continual communication with exhaust by way of a port which is capable of being closed by a pumping piston which partly defines said variable volume chamber when the said pumping piston is at its end of stroke in a sense to contract said variable volume chamber, so that, when said port in the variable volume chamber is closed by said pumping pis n, fluid is trapped in the said variable volume chamber and said trapped fluid acts to arrest the said pumping piston at its said end of stroke.

13. A motor as claimed in claim 7 wherein the spool when moving in at least one sense of axial direction is adapted to displace fluid from an arresting chamber by way of a port which is capable of beingclosed by the spool at its end of stroke in said one sense of axial direction so that, when said port in the arresting chamber is closed by the spool, fluid is trapped in the said arresting chamber and said trapped fluid acts to arrest the spool at its said end of stroke.

14. A motor as claimed in claim 13 in which the spool has two spool rods, one at each end thereof, each spool rod being axially slidable in a blind bore, and wherein two arresting chambers are provided, one at each end of the spool, the arresting chambers being formed between the ends of the spool rods and the- {closed end part lengths of the blind bores in which the spool rods are respectively slidable, the arresting chambers each having a port which communicates with exhaust and is controlled by the respective spool rod to be closed to trap fluid in that arresting chamber immediately prior to the spool reaching its end of stroke in a sense to contract that arresting chamber so that the spool is arrested at both ends of its stroke by the fluid trapped in the respective arresting chambers.

15. A motor as claimed in claim 1 wherein fluid pressure flow into, and fluid displaced from, the alternating pressure piston chamber is by way of two ports in the piston cylinder, one of said two ports being restricted and located to be in permanent communication with the alternating pressure piston chamber and the other of said two ports being located to be closed by the piston at its end of stroke in a sense to contract the alternating pressure piston chamber so that when the said other of said two ports is closed by the piston as the alternating pressure piston chamber is contracted, fluid is displaced from the alternating pressure piston chamber solely by way of the said restricted port which imparts a throttling action on said displaced fluid and provides a degree of arresting on the piston at its end of stroke in said sense to contract the alternating pressure piston chamber.

16. A percussive device which includes a motor as claimed in claim 1 and wherein said device comprises a socket which is substantially co-axial with the piston and is adapted to removably and slidably receive a shank of a tool to be impacted and the piston is adapted to impart intermittent blows to said tool. 

1. A fluid pressure operated reciprocating motor which includes; a pressure biased piston axially slidable in a piston cylinder, the piston having different effective areas of which the larger effective area defines with one part of the piston cylinder an alternating pressure piston chamber and the smaller effective area defines with another part of the piston cylinder a constant pressure piston chamber which constant pressure piston chamber is in permanent communication with a fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the piston is biased for movement in one sense of axial direction; valve means adapted to connect alternately the alternating pressure piston chamber with fluid pressure inlet and exhaust to reciprocate the piston when the inlet is connected to a source of fluid under pressure, said valve means further being adapted to open communication between the constant pressure piston chamber and the alternating pressure piston chamber when the piston is moving in the direction against its bias so that fluid displaced from the constant pressure piston chamber is fed back into the alternating pressure piston chamber; a variable volume chamber which is in substantially continual communication with exhaust and is controlled by the piston so as to expand or contract as the case may be when the piston moves in the direction of or against its bias respectively; and wherein the said valve means acts to close and open communication alternately between the alternating pressure chamber and the said variable volume chamber so that exhaust fluid from the alternating pressure piston chamber can enter the expanding said variable volume chamber when the piston is moving in the direction of its bias and that, when the piston is moving in the direction against its bias, fluid from the contracting said variable volume chamber is delivered directly to exhaust to alleviate intermittent exhaust back pressure in the motor.
 1. A fluid pressure operated reciprocating motor which includes; a pressure biased piston axially slidable in a piston cylinder, the piston having different effective areas of which the larger effective area defines with one part of the piston cylinder an alternating pressure piston chamber and the smaller effective area defines with another part of the piston cylinder a constant pressure piston chamber which constant pressure piston chamber is in permanent communication with a fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the piston is biased for movement in one sense of axial direction; valve means adapted to connect alternately the alternating pressure piston chamber with fluid pressure inlet and exhaust to reciprocate the piston when the inlet is connected to a source of fluid under pressure, said valve means further being adapted to open communication between the constant pressure piston chamber and the alternating pressure piston chamber when the piston is moving in the direction against its bias so that fluid displaced from the constant pressure piston chamber is fed back into the alternating pressure piston chamber; a variable volume chamber which is in substantially continual communication with exhaust and is controlled by the piston so as to expand or contract as the case may be when the piston moves in the direction of or against its bias respectively; and wherein the said valve means acts to close and open communication alternately between the alternating pressure chamber and the said variable volume chamber so that exhaust fluid from the alternating pressure piston chamber can enter the expanding said variable volume chamber when the piston is moving in the direction of its bias and that, when the piston is moving in the direction against its bias, fluid from the contracting said variable volume chamber is delivered directly to exhaust to alleviate intermittent exhaust back pressure in the motor.
 2. A motor as claimed in claim 1 wherein the variable capacities of the constant pressure piston chamber, the alternating pressure piston chamber and the said variable volume chamber are so related that when the alternating pressure piston chamber is expanding, the volume of fluid displaced from the constant pressure piston chamber in a given time is substantially equal to the volume of fluid entering the alternating pressure piston chamber less the volume of fluid displaced from the said variable volume chamber in that time; and that when the alternating pressure piston chamber is contracting the volume of fluid entering the constant pressure piston chamber in a given time is substantially equal to the volume of fluid displaced from the alternating pressure piston chamber less the volume of fluid entering the said variable volume chamber in that time.
 3. A motor as claimed in claim 1 wherein the said variable volume chamber is defined by a secondary piston and cylinder device which device is operatively connected to the main piston and cylInder device which defines the constant and alternating pressure piston chambers to provide required sequential expansion and contraction of the said variable volume chamber in accordance with expansion and contraction of the constant pressure piston chamber during axial movement of the main piston in its cylinder.
 4. A motor as claimed in claim 1 wherein the piston defines with its cylinder the said variable volume chamber.
 5. A motor as claimed in claim 4 wherein the piston cylinder has a stepped bore and the piston carries two piston lands, the first, which is located in the larger bore of the piston cylinder, has differential effective end areas and effectively constitutes a part of the pressure biased piston proper, and the second, which is located in the smaller bore of the piston cylinder, effectively constitutes part of a pumping piston; and wherein the alternating pressure piston chamber is formed between the end of the larger piston cylinder bore and the outwardly facing end of the first piston land, the said variable volume chamber is formed between the end of the smaller piston cylinder bore and the outwardly facing end of the second piston land, and the continuous pressure piston chamber is constituted by the annulus defined by the wall of the stepped bore between the inwardly facing end of the first piston land and the opposed face of the second piston land.
 6. A motor as claimed in claim 1 wherein the said valve means is provided by a single valve which is operatively controlled by movement of the piston and is adapted to alternately open and close communication between the alternating pressure piston chamber and the constant pressure piston chamber and concurrently to respectively close and open communication between the said variable volume chamber and the alternating pressure piston chamber.
 7. A motor as claimed in claim 1 wherein the said valve means comprises a spool which reciprocates in a spool cylinder and in which the spool and piston mutually control reciprocation of each other in their respective cylinders.
 8. A motor as claimed in claim 7 wherein a land of the spool controls the opening and closing of a port in the spool cylinder which is in permanent communication by way of a piston signalling passage with the alternating pressure piston chamber and movement of the spool is operatively associated with movement of the piston so that when the spool is located at one end of its stroke the piston signalling passage communicates by way of the spool cylinder with fluid pressure inlet whereby the piston is caused to move in the direction against its bias and when the spool is located at the other end of its stroke the piston signalling passage communicates by way of the spool cylinder both with exhaust and with the said variable volume chamber whereby the piston is caused to move in the direction of its bias.
 9. A motor as claimed in claim 7 wherein the said valve means comprises a pressure biased spool axially slidable in the spool cylinder, the spool having different effective areas of which the larger effective area defines with one part of the spool cylinder an alternating pressure spool chamber and the smaller effective area defines with another part of the spool cylinder a constant pressure spool chamber which constant pressure spool chamber is in permanent communication with fluid pressure inlet so that when the inlet is connected to a source of fluid under pressure the spool is biased for movement in one sense of direction; and wherein the alternating pressure spool chamber is adapted to communicate by way of spool signalling passage means with port means controlled by movement of the piston which port means is, during reciprocation of the piston, controlled to communicate alternately with fluid inlet and exhaust so that the alternating pressure spool chamber is alternately pressurized and exhausted to reciprocate the spool.
 10. A motor as claimed in claim 9 wherein the port means of the spool signalling passage means is controlled by a land of the piston so that at one end of the stroke of the piston said port means communicates with fluid inlet and at the other end of the stroke said port means communicates with exhaust by way of the said variable volume chamber.
 11. A motor as claimed in claim 9 wherein at one end of stroke of the piston, the port means of the spool signalling passage means communicate with fluid inlet by way of the constant pressure piston chamber.
 12. A motor as claimed in claim 1 wherein the said variable volume chamber is in substantially continual communication with exhaust by way of a port which is capable of being closed by a pumping piston which partly defines said variable volume chamber when the said pumping piston is at its end of stroke in a sense to contract said variable volume chamber, so that, when said port in the variable volume chamber is closed by said pumping piston, fluid is trapped in the said variable volume chamber and said trapped fluid acts to arrest the said pumping piston at its said end of stroke.
 13. A motor as claimed in claim 7 wherein the spool when moving in at least one sense of axial direction is adapted to displace fluid from an arresting chamber by way of a port which is capable of being closed by the spool at its end of stroke in said one sense of axial direction so that, when said port in the arresting chamber is closed by the spool, fluid is trapped in the said arresting chamber and said trapped fluid acts to arrest the spool at its said end of stroke.
 14. A motor as claimed in claim 13 in which the spool has two spool rods, one at each end thereof, each spool rod being axially slidable in a blind bore, and wherein two arresting chambers are provided, one at each end of the spool, the arresting chambers being formed between the ends of the spool rods and the closed end part lengths of the blind bores in which the spool rods are respectively slidable, the arresting chambers each having a port which communicates with exhaust and is controlled by the respective spool rod to be closed to trap fluid in that arresting chamber immediately prior to the spool reaching its end of stroke in a sense to contract that arresting chamber so that the spool is arrested at both ends of its stroke by the fluid trapped in the respective arresting chambers.
 15. A motor as claimed in claim 1 wherein fluid pressure flow into, and fluid displaced from, the alternating pressure piston chamber is by way of two ports in the piston cylinder, one of said two ports being restricted and located to be in permanent communication with the alternating pressure piston chamber and the other of said two ports being located to be closed by the piston at its end of stroke in a sense to contract the alternating pressure piston chamber so that when the said other of said two ports is closed by the piston as the alternating pressure piston chamber is contracted, fluid is displaced from the alternating pressure piston chamber solely by way of the said restricted port which imparts a throttling action on said displaced fluid and provides a degree of arresting on the piston at its end of stroke in said sense to contract the alternating pressure piston chamber. 